Because the result of any arithmetic comparison with
NULL is also NULL, you
cannot obtain any meaningful results from such comparisons.

In MySQL, 0 or NULL
means false and anything else means true. The default truth
value from a boolean operation is 1.

This special treatment of NULL is why, in
the previous section, it was necessary to determine which
animals are no longer alive using death IS NOT
NULL instead of death <>
NULL.

Two NULL values are regarded as equal in a
GROUP BY.

When doing an ORDER BY,
NULL values are presented first if you do
ORDER BY ... ASC and last if you do
ORDER BY ... DESC.

A common error when working with NULL is to
assume that it is not possible to insert a zero or an empty
string into a column defined as NOT NULL,
but this is not the case. These are in fact values, whereas
NULL means “not having a
value.” You can test this easily enough by using
IS [NOT] NULL as shown: